The present invention relates generally to modified polycarbonate compositions with improved impact and environmental stress crack resistance properties. More particularly, the present invention relates to blends of aromatic polycarbonates and high molecular weight hydrogenated styrene-butadiene-styrene triblock copolymers.
Aromatic polycarbonates are well known commercially available materials having a variety of applications in the plastics art. Generally speaking, these resins offer high resistance to attack by mineral acids, have high tensile strength and high impact strength, except in thick sections, good thermal resistance and a dimensional stability far surpassing that of most other thermoplastic materials.
The use of aromatic polycarbonate resins in certain applications is limited, however, because they have a high viscosity in the melt, making molding of complex large and especially formed parts difficult. They also exhibit brittleness under sharp impact conditions in thick section and, regardless of thickness, when small amounts of reinforcements such as glass, or pigments such as titanium dioxide, are added for conventional purposes. In addition, polycarbonate resins exhibit severe environmental stress cracking. The term "environmental stress cracking" refers to the type of premature failure under stress which is hastened by the presence of organic solvents, e.g., acetone, heptane and toluene, when such solvents are in contact with stressed articles fabricated from aromatic polycarbonate resins. Such contact may occur, for example, when solvents are used to clean or degrease stressed parts fabricated from polycarbonates, or when such parts are used around gasoline engines in automotive and recreational applications.
As a result, polycarbonate polymers have been modified by or blended with additional polymers to achieve materials with the desired combination of properties. For example, in U.S. Pat. No. 4,088,711 polycarbonates are combined with block copolymers of a monoalkenyl arene polymer and a completely hydrogenated conjugated diene to form a continuous interlocking network, with the block copolymer acting as a mechanical structural stabilizer. The compositions described in U.S. Pat. Nos. 4,537,930, 4,267,096 and 4,122,131 are blends of polycarbonates and small amounts of vinyl aromatic and olefin elastomer copolymers having improved environmental stress crack resistance. U.S. Pat. No. 4,628,072 describes the improvement of physical properties such as adhesion, impact resistance, weatherability and heat resistance by the addition of block copolymers of a monovinyl substituted aromatic hydrocarbon polymer and unsaturated olefin compound polymer to various thermoplastic polymers. See also U.S. Pat. No. 4,579,903 directed to copolyester carbonate resin compositions incorporating copolymers of vinyl aromatic compounds and olefinic elastomers which provide improved solvent resistance and impact properties.
The present invention is directed toward novel polycarbonate compositions having markedly improved environmental stress crack resistance as well as improved impact resistance properties.